Trademark. The protection is limited unless you can establish a public association with your name that would cause confusion if someone else used it. It can’t be used fraudently of course.
Besides being localized, a DBA isn’t unique and doesn’t assert ownership. If I register as dba McDonald’s, I’m not claiming any ownership of McDonald’s as a name. I’m just announcing to the world that I’m operating under that name. In a case like this, I’m probably just a franchisee with contractual permission to use the trademark.
Some business names can be used over and over. You can have a Joe’s Diner in every city. If you want a domain name, you can get joesdinerchicago or joesmadisonavediner or any other variation.
You can also reuse names across businesses. There was a Cadillac dog food as well as the Cadillac cars.
Trademark law allows this, because trademarks are very specific. Despite popular opinion, you can’t trademark names, just uses of names.
“How do you protect a business name?” isn’t an answerable question. This is one of many areas in which the details are all-important.
You can protect a corporate name simply by registering it as the name of the corporation. States will not allow a corporation to register a name already in use, unless the incumbent has dissolved. That doesn’t protect you from people in other states, of course, but you can always register a subsidiary under the same name in each state.
You want a trade mark. Full stop. The other registrations are not “protections”, they are more like “notifications”. Anyone else with a trade mark can stop you. If I went and registered mcd0nalds.com, you bet you hiney McDonald’s Inc would come after me, and the stick they would hit me with is their registered trademark “McDonald’s”.
Isn’t there a real case where some poor bastard named McDonald opened up a shop with his name and got steamrolled by the golden arches?
It might have been a burger diner, or something, so the guy was not so innocent (unless really was from Mars and never heard of the other company), but still.
The McDonald brothers, after selling their original restaurant (and rights to the name) to Roy Kroc, after seeing how successfully he was spreading the chain, tried to open another restaurant, again using their name, McDonalds. Even though it was not a fast food type, they were still sued, and lost.
But trademarks are not self-enforcing.
Next you have to be prepared to fight off anyone who attempts to infringe on your trademark. The trademark office is not going to do this for you. You have to keep vigilant and then hire and pay your own lawyers to scare off any infringers.
Registering the domain, by itself, isn’t going to do any good. If someone has a legitimate claim that the domain name infringes their trademark, they can take steps to take it away from you. But if you want to fight for it, there are advantages to being first.
If you are serious, you need to talk to an attorney who specialized in IP law.